Thursday, May 8, 2014

New paper finds transient climate sensitivity to CO2 is ~35% less than IPCC claims

A paper under discussion for Earth System Dynamics finds the transient climate response [TCR] to CO2 is 1.3°C per doubling of CO2 levels, about 35% less than claimed by the IPCC mean estimate and the same as determined by another recent paper by Otto et al finding a TCR of 1.3°C. The authors

"assess the origin of these differences [between the IPCC high TCR estimates and lower estimates from this paper and others] and highlight the inverse relation between the derived anthropogenic temperature trend of the past 30 years and the weight given to the [natural] Atlantic Multidecadal Oscillation (AMO) as an explanatory factor in the multiple linear regression (MLR) tool that is usually employed. We highlight that robust MLR outcomes require a better understanding of the AMO in general and more specifically its characterization. Our results indicate that both the high- and low end of the anthropogenic trend over the past 30 years found in previous studies are unlikely and that a transient climate response with best estimates centred around 1.3°C per CO2 doubling best captures the historic instrumental temperature record."

G. R. van der Werf and A. J. DolmanVU University Amsterdam, Faculty of Earth and Life Sciences, Amsterdam, the NetherlandsAbstract. The instrumental surface air temperature record has been used in several statistical studies to assess the relative role of natural and anthropogenic drivers of climate change. The results of those studies varied considerably, with anthropogenic temperature trends over the past 25–30 years suggested to range from 0.07 to 0.20 °C decade−1. In this short communication we assess the origin of these differences and highlight the inverse relation between the derived anthropogenic temperature trend of the past 30 years and the weight given to the [natural] Atlantic Multidecadal Oscillation (AMO) as an explanatory factor in the multiple linear regression (MLR) tool that is usually employed. We highlight that robust MLR outcomes require a better understanding of the AMO in general and more specifically its characterization. Our results indicate that both the high- and low end of the anthropogenic trend over the past 30 years found in previous studies are unlikely and that a transient climate response with best estimates centred around 1.3 °C per CO2 doubling best captures the historic instrumental temperature record.

5 comments:

When the fact that the AMO seems to be driven by solar activity is taken into account the amount of warming that can be attributed to natural solar/AMO effects becomes greater still, reducing the sensitivity estimate still further, but the more important implication is for projections of global temperature change, which shift from increasing to decreasing.

The paper is now out of the discussion phase and published, please see this link. Addressing some of the reviewer's comments and updating the datasets to include other temperature datasets and updated forcings the transient climate response was calculated as 1.6 degrees with an uncertainty raten of 1.0-3.3 in the revised version. All code used is available, please see the link in the paper.

This gives in combination with the IPCC RCP8.5 scenario where we are currently tracking for CO2 (but we are below for CH4) a warming of 2.9 (1.5-6.8) degrees over current temperatures, all else being equal. Of course all else is not equal and things can be better or worse but it is difficult to use these studies which point to relatively low climate sensitivity as an argument to not take climate change serious.

Why do you disagree? Have you ever done the math, calculating temperature change for the business-as-usual RCP8.5 scenario for example? Even with the median value from the link you gave one exceeds 2 degrees then. And there are many papers in that list that do not even mention climate sensitivity, especially the ones with lower values, so you may have to consider that your median value is biased low.

1. RCP is not business as usual and makes unrealistic assumptions on emissions and population growth, etc.2. RCP 6 is "business as usual"3. The climate sensitivity can be calculated from each of the papers listed4. I'd say the median value is biased high, because my own calculations of climate sensitivity based upon observations show 0.25-0.28 CS